The Pentagon says Iranian-backed militias fighting Islamic State (IS) extremists in Syria used Abrams tanks that the U.S. military had originally provided to the Iraqi Army.

However, Pentagon spokesman Eric Pahon said on February 9 that the estimated nine tanks were later returned to the Iraqi military.

The Iranian-backed Shi'ite Popular Mobilization Forces (PMF) are legally considered by Iraq to be part of the Iraqi Security Forces, Pahon said. But he added that the United States had never supplied "defense articles" to the militia.

"We have discovered incidents where some U.S.-origin equipment, including M1 Abrams tanks, came into the possession of certain PMF groups. This equipment was subsequently used by these groups" in the fight against the IS extremists.

"All of these tanks were recently returned to Iraqi Security Forces custody," Pahon added.

Iraqi Kurdish officials contend that the Shi'ite paramilitary groups used Abrams tanks against Sunni Kurds in northern Iraq last year. That led U.S. officials to threaten to curtail military aid to Baghdad until the tanks were returned to Iraq control, a quarterly report by the U.S. Inspector-General said.

The Sunni-led IS fighters seized large parts of Syria and Iraq in 2014, declaring an Islamic "caliphate" and committing widespread atrocities.

But they have been steadily pushed out of nearly all of the territory they had controlled in both countries.

The United States has approved sales of more than $22 billion in arms and equipment, including at least 150 Abrams tanks, to Iraq since 2005.